1. Field of the Invention
The present invention relates, in general, to light-transmissive polycrystalline alumina ceramics mainly used for a light-transmissive alumina luminous tube of high voltage sodium lamp and, more particularly, to a light-transmissive alumina ceramic composition and a method for preparing the same, capable of improving the light-transmission and the workability of polycrystalline alumina ceramics.
2. Description of the Prior Art
Since light-transmissive alumina ceramics, in general, show good transmissivity to visible lights and infra-red lights and have superior or thermal, mechanical, chemical properties and preferred electric stability, they are mainly used as materials for luminous tube of high voltage lamp and for window for infra red sensor.
The light transmissivity of polycrystalline alumina ceramics is determined by its surface roughness, size and uniformity of crystalline particle, void ratio, and impurity content. That is to say, a light passing through polycrystalline alumina ceramics reflects or scatters at the surface thereof, the surface of particle constituting it, the void and the impurity contained therein, and as the path of the light becomes lengthy due to the reflection and scattering, there is effected lower transmissivity of light, consequently.
In order to improve the light transmissivity, many efforts have been made to prevent the reflection and scattering. For example, the reflection and scattering caused by particle surface, void and impurity may be reduced by using highly pure raw material and by controlling additives and sintering conditions. In addition, the reflection and scattering at the surface of ceramics can be prevented by smoothly polishing the surface of ceramics.
As methods for polishing light-transmissive alumina ceramics, there have been used two types, that is, a chemical polishing method and a mechanical polishing method.
The chemical polishing method, which was developed by G. E. Scott, Jr, et al, comprises of treating light-transmissive alumina ceramics in a sodium borate solution having temperatures ranging from 762.degree. to 857.degree. C., removing impurities attached to the surface of the alumina ceramics with fluoric acid (HF) and drying it in the air, so as to obtain smooth surface.
However, the chemical polishing method is difficult to apply to mass production for an article of complex shape, such as light-transmissive alumina luminous tube for high voltage sodium lamp, since it is complicate and requires much time.
On the other hand, the mechanical polishing method comprises of applying an grinding process and a cutting process to the surface of the sintered, light-transmissive alumina ceramics and polishing mechanically to smooth the surface.
Since the sintered substance of alumina ceramics is extensively hard, considerable load is needed to carry out the mechanical polishing method. This considerable load may damage the alumina ceramics and effect minute cracks therein, so that it acts as a factor that causes poor qualities in an article produced, such as deterioration of article reliability.